Season whisked eggs with salt and pepper. Wipe skillet clean and return to medium heat. When hot, swirl in cooking oil. Add in the eggs and cook until the bottom is just barely set. Run the spatula along the edge and bottom of egg, to loosen. Flip omelet over, then spoon on the mushroom and spinach mixture to one side. Add in the cheese and fold over the omelet. Cook until omelet is cooked through and cheese melted. Top with fresh tomatoes.